Inkjet printed IGZO memristors with volatile and non-volatile switching

Solution-based memristors deposited by inkjet printing technique have a strong technological potential based on their scalability, low cost, environmentally friendlier processing by being an efficient technique with minimal material waste. Indium-gallium-zinc oxide (IGZO), an oxide semiconductor material, shows promising resistive switching properties. In this work, a printed Ag/IGZO/ITO memristor has been fabricated. The IGZO thickness influences both memory window and switching voltage of the devices. The devices show both volatile counter8wise (c8w) and non-volatile 8wise (8w) switching at low operating voltage. The 8w switching has a SET and RESET voltage lower than 2 V and − 5 V, respectively, a retention up to 105 s and a memory window up to 100, whereas the c8w switching shows volatile characteristics with a low threshold voltage (Vth < − 0.65 V) and a characteristic time (τ) of 0.75 ± 0.12 ms when a single pulse of − 0.65 V with width of 0.1 ms is applied. The characteristic time alters depending on the number of pulses. These volatile characteristics allowed them to be tested on different 4-bit pulse sequences, as an initial proof of concept for temporal signal processing applications.


Influence of IGZO thickness on resistive switching characteristics
To transition into a sustainable economy, it is critical to minimize the waste.In this work, to print a 1 mm 2 square, only 10 µL of IGZO precursor ink were consumed.In this section the focus will be on the optimization of the IGZO printing and the impact on layer thickness and consequently its resistive switching characteristics.
First, to achieve a good quality printed IGZO layer the influence of UV surface treatment before printing each layer was studied.One of the issues in uniformity of inkjet printed films is the coffee ring effect.One approach to mitigate this effect is to heat the substrate during printing 49 , which was adopted in this work.Without any surface treatment, the profilometer data summarized in Fig. 1b and in Figure S1 shows the printed IGZO layer has a low coffee ring and a good coverage without the existence of deep valleys.It is found, that after drying the first IGZO layer, its dimensions shrink 25% to 0.75 × 0.75 mm.Moreover, the IGZO thickness does not scale in a linear trend in function of the number of layers.One printed layer has a thickness of 141 ± 42 µm whereas 5 printed layers have a thickness of 572 ± 181 µm, about 4 times higher than a single layer.The atomic force microscopy (AFM) measurements, shown in Figure S2, are in agreement with the profilometer data.For 1 layer of IGZO, it shows an average thickness of 115 ± 23 nm with an average roughness of 20 ± 5 nm.The average roughness increases to a maximum of 45 nm for 5 printed layers of IGZO.The inset of Fig. 1b shows a microscope image of the printed Ag/IGZO/ITO memristors.The printing definition is great with well-defined borders.On the other hand, when 15 min of UV were applied before printing the IGZO layer, the film shows noticeable overspreading, thus reducing the thickness of the printed IGZO films.The thickness of 1 and 5 layers are 15 ± 10 µm to 319 ± 40 µm, respectively (Figure S3), a respective decrease of 90% and 45% when compared to the non-treated films.
Figure 1c shows the influence of the device characteristics in function of the IGZO thickness.Regardless the thickness, the devices show gradual switching in both SET and RESET.Moreover, the devices became more resistive with the increase of the thickness (Figure S4).For an IGZO thickness between 50 and 620 µm, the average read current drops from 1.7 ± 0.2 mA to 0.15 ± 0.09 mA for LRS and from 0.3 ± 0.1 mA to 0.7 ± 0.5 µA for HRS (Fig. 1d).The HRS current decreases with IGZO thickness increment.Also, the memory window, ratio between the LRS and HRS current, increases in an exponential trend from 5 up to 200 (Fig. 1e,h).
Figure 2(a,d) shows the endurance curves of Ag/IGZO/ITO memristors with different IGZO thickness with respective current levels when reading at 100 mV Fig. 2(e,h).The devices show stable switching with low dispersion.Both SET and RESET voltages increase with the increase of IGZO thickness, from 1 to 2 V and from − 1.5 V to − 5 V for SET and RESET, respectively.There is a decrease in cycle variation with the decrease of thickness.Moreover, the switching became more gradual with the decrease of thickness.However, the trade-off is the decrease of memory window.Also, the IGZO devices with the lowest thickness (50 nm) show an increase of the HRS current during cycling, thus gradually reducing the memory window during endurance test.For the thicker devices that trend in HRS current does not occur.Moreover, the existence of a secondary volatile switching with opposite polarity was found (Figure S5).
To study the switching mechanism of the Ag/IGZO/ITO memristors, temperature measurements were performed in vacuum from 150 to 300 K.As shown in Fig. 3a, there are no significant changes in current at LRS at 150 K and 300 K, but HRS displays a more pronounced change with temperature.Figure 3b,d show the best current-voltage and current-temperature fittings for LRS.LRS charge transport is assumed to be controlled by variable range hopping (i.e., temperature dependency of 1/T 4 ) which is usually reported for strongly disorder systems 50,51 .It is bulk-limited model indicating that defects in IGZO have important roles.The slope double logarithmic plot is 1.01, implying ohmic characteristics as dominated conduction.Therefore, the origin of defects is most probably related to diffusion of silver ions in filament formation.At HRS, the characteristics show a good fit of Schottky emission (thermionic emission) with a good quality fit when plotting the ln (I/V) in function of V 1/2 also validated by temperature dependency fitting as shown in Fig. 3e.

Proof of concept of volatile and non-volatile switching characteristics
From the results of the previous section, IGZO with a thickness of 50 nm was chosen for this proof of concept because it leads to a more gradual switching and a lower variability when cycling the devices.These characteristics are important for neuromorphic computing.The optimized fabricated memristors (the device structure and connection depicted in Fig. 4a, show two distinctive I-V characteristics according to the endurance and current-time relation measurements.The distinct switching mechanisms are explained in the discussion section.There is a significant difference in current levels  between two switching modes (Fig. 4b), meaning that only a certain level of current defines resistive switching modes.By standard definition, the electroforming process is the one-time application of a high electric field, higher than the set voltage (V f > V set ).In the presented devices, in both volatile and non-volatile modes, there is no significant difference between initial SET and other cycles.This way, the device can be categorized as formingfree.In previous works 18,19,52,53 , the IGZO memristors presented a forming-free performance.The devices in their pristine state have a low current (ranging from 10 -10 A to 10 -5 A) and a rectification of 1000 (Figure S6).Using the terminology originated from the works of Dittmann and Waser [54][55][56] , the switching polarity can be classified into counter-eightwise (c8w) and eightwise (8w) in relation to the active electrode.When the voltage of the active electrode is displayed and the voltage of the other electrode is grounded, then the switching polarity will be called c8w if the SET occurs at negative voltage and reset occurs at positive voltage.The c8w I-V curve (in a linear scale) has a drawing direction which is against that of the handwriting of a (tilted) '8' .The opposite switching polarity is called 8w 55 .In this work the voltage is applied to the Ag electrode whereas the ITO electrode is grounded.Since the Ag is a more reactive electrode than ITO, thus we considered the Ag as the active electrode (AE).Therefore, the 8w switching has non-volatile bipolar nature where the c8w switching have volatile characteristics.
In Fig. 4c, the current follows an 8w pinched hysteresis loop with bipolar non-volatile properties (the linear I-V characteristic is shown in Figure S1).The device is compliance free, reaching currents up to 50 mA with a rectification feature.The SET voltage is at 0.8 V while the RESET voltage shows a higher variation being between − 0.5 and − 0.9 V.In the lower voltage regime, however, the direction of SET and RESET are reversed demonstrating so-called c8w switching and volatile resistive switching is obtained.The volatile behaviour shows a very low cycle-to-cycle variability during endurance and a low threshold voltage between − 0.2 V and − 0.3 V (Fig. 4d) with a rectification in the ratio of 100 similar to our previous report 18 using sputtered IGZO device.Only the c8w switching behaviour shows retention time as shown in Fig. 4e for 10 5 s.
The short-term memory effect of the memristor for the volatile regime can be described by a time constant τ, from an exponential decay function.Figure 4f depicts the decay curve after a − 0.65 V pulse for 100 µs.The relaxation time, τ, is 0.75 ms.As a result, when programming the device, the device state not only depends on the programming pulse itself, but also on the number of pulses and the pulse intervals.
To demonstrate the similitude amongst the dynamic memory retention of the device and that of the human memory, a single stimulus (− 0.65 V with a duration of 0.1 ms) spaced by a period greatly larger than τ (Fig. 5a) The pulses have an amplitude of − 0.65 V with a duration of 0.1 ms.The results are described in Fig. 5b where a good quality fitting is also presented.Similarly to the results from literature 44 , the current decay follows a simple exponential decay function.Both the relaxation time constant, τ, and the initial current increase with the increasing number of pulses, suggesting that the dynamic retention can be increased by repeating stimulations.The τ increases from 0.75 ms to 11 ms (Fig. 5c) and the initial current increases from 1.8 µA to 2.5 µA.
Figure 5d shows the effect of the interval between pulses.After the stimulation, the higher is the interval between pulses, the lower is the read current.For a pulse interval of 0.1 ms, the current reaches 1.5 µA after 7 pulses, whereas for a pulse interval of 1 ms, the current only reaches 0.5 µA.Since the application for temporal signal processing requires very short-term memory, volatile memristors like the one presented in this work are a great potential candidate.Figure 6a shows the volatile mode memristor device characteristics to different temporal inputs.
The "1" state corresponds to a pulse with − 0.65 V of amplitude and a width of 100 µs.The "0" state corresponds to the absence of pulses, 0 V amplitude for 1 s.There are 5 read pulses: one when the sequence initiates and then after finishing each state.Figure 6b shows that a pulse width variation of 10 µs, 50 µs and 100 µs can be used to activate the device.The longer the pulse, the higher is the corresponding read current.Also, when applying different pulse voltages (− 0.35 V, − 0.5 V, − 0.65 V), the read current is higher (Fig. 6c).
When a pulse is applied, the state of the memristor will be changed by increasing its conductance and if the pulse interval is short enough, its conductance will be increased.For long intervals the conductance decays to its resting state 43 .Therefore, different temporal inputs will lead to different states of the device.[0110] and [0101] sequences, using the same "0″ and "1″ pulse parameters depicted in Fig. 6a.There is a low variation of the read current over the cycles, however, the trend is the desired one.
The working devices are very consistent in terms of voltage operation (Figure S8a), the same pulse scheme works for the different devices as can be shown in Figure S8b.However, they present some variability regarding the current state due to the presence of pinholes.We also note that reducing the size of the active region can lead to faster switching times, therefore shorter and more intense pulses may induce faster switching.

Discussion
The devices show a co-existence of threshold switching volatile memristor and bipolar non-volatile switching.The devices can alter from volatile mode (c8w) to non-volatile (8w), but not vice versa.The protocol to change from volatile to non-volatile is by increasing the switching voltage without needing electroforming.The switching polarity is related to the dominated means of the defect redistribution.In VCM filamentary systems, the c8w switching mode the device is SET to its low resistive state (LRS) by applying a negative voltage at the AE and RESET to its high resistive state (HRS) by applying a positive voltage at the AE.The second resistive switching, 8w, occurs at the opposite polarity of the c8w mode.In 8w switching a positive voltage is applied to the AE of the device to bring it to its LRS and a negative voltage is needed to RESET the device to its HRS 57 .On oxide thin film memristors, it has been demonstrated that both switching modes can appear in the same device by changing the operating conditions 54,56,[58][59][60][61] .In the context of non-volatile 8w switching, the temperature and voltage characteristics suggests that the charge transport in the low-resistance state (LRS) is governed by the variable range hopping model, which is bulk-limited.The electrons are injected into the IGZO without significant potential barrier, and the transport-limiting element is the conduction from defect to defect.Non-volatile switching with silver as active electrode is usually due to metallic cation migration, recognized as electrochemical metallization mechanism (ECM) 62 .This is illustrated in the schematic shown in Fig. 4a.The explanation for the increase in current with increasing temperatures is the nanostructured morphology of the filaments 63 .
The rectification characteristics on the I-V curves on both switching polarities, are due to the presence of small Schottky-type barriers at the interface of Ag/IGZO layers and ITO/IGZO layers for non-volatile 8w switching, and volatile c8w switching respectively.On non-volatile switching at HRS, the characteristics shown a good fit of Schottky emission (thermionic emission).This means that the conduction in the non-volatile HRS is supported by the conduction band of the IGZO and the transport-limiting element is the injection of electrons at the contact interface.
The coexistence of c8w and 8w switching was reported in 58 for Pt/TiO 2 /Ti/Pt devices where both switching modes occur from the competition between drift/diffusion of oxygen vacancies in the oxide layer and an oxygen exchange reaction across the Pt/TiO 2 interface.A similar concept can be applied here for the c8w resistive switching, categorized as diffusive memristor following ion exchange at the interface of IGZO and ITO.We have already shown in our previous works 18,19,52,53 , that amorphous oxide semiconductors (AOS)-based memristor present a forming-free performance.One of the main reasons relies on defect profiles of AOS active material at the interface with the electrode which can be easily tuned into distinct resistance states especially in c8w Figure 6.(a) 1100 pulse stream with the optimized parameters: for the state 1 it was applied a − 0.65 V pulse for 0.1 ms; the state 0 is 1 ms after the last "1" pulse; the reading was performed at 0.05 V for 1 ms.(b) Influence of the pulse length for the "1" state (0.1 ms, 0.5 ms and 0.01 ms) using a [1100] pulse stream.(c) Influence of the pulse amplitude for the "1" state (− 0.35 V, − 0.5 V and − 0.65 V) using a [1100] pulse stream (d) Read current for various pulse streams using the "1" and "0" pulse parameters in a). Vol:.(1234567890)

Scientific Reports
| (2024) 14:7469 | https://doi.org/10.1038/s41598-024-58228-ywww.nature.com/scientificreports/resistive switching behaviour as shown in one or our previous works 64 .The corresponding resistive switching is area-dependent; however, multi-filamentary nature is not excluded.Moreover, the coexistence of secondary switching in a single memristor cell is usually volatile as reported in 53,56,58 , 65,66 .In these works, the volatile switching mode is explained by an oxygen exchange reaction between the Pt electrode at the interface with active layer, e.g.metal-oxide.The occurrence of volatile mode at negative voltage polarity laine may be related to ion-related migration at the interface 41 .The exchange of oxygen between ITO and the switching layer can influence the conductivity of the latter 67 .In case of an n-type material like IGZO, the conductivity will increase with the decrease of oxygen content.Hence, under positive polarity at the top electrode, oxygen moves into the ITO layer and gets accommodated as interstitial oxygen, which corresponds to the SET operation.This interstitial oxygen is released back into the IGZO under positive bias during the RESET operation (see Fig. 4a).

Conclusions
It is demonstrated that solution-based memristors fabricated by inkjet technique have a strong potential for applications due to their scalable production at low cost and low waste formation.In this work, a printed IGZO memristor has been fabricated where only 10 µL of IGZO precursor ink was spent to print a 1 mm 2 square with minimum waste.The devices shows both volatile and non-volatile behaviour depending on the programming schemes.The IGZO thickness influences the switching voltage and memory window.The non-volatile response follows an 8w switching polarity with a SET and RESET voltage higher than 2 V and − 5 V, respectively, with low cycle variability and a retention up to 10 5 s and a memory window up to 100.The LRS charge transport is found to be controlled by variable range hopping where the origin of defects on IGZO is most probably related to the diffusion of silver ions in the form of filaments.On the other hand, the volatile switching mode follows an 8w scheme with very low threshold voltage (V th < − 0.65 V) and switching times below 1 ms.The volatile characteristics provide short term retention with a τ of 0.75 ms.Those combined characteristics show that a low-cost technology like printed metal oxide memristors can be used for simple and efficient designs of fully memristive architecture based on IGZO, where the reservoir state (volatile mode) can be processed with the IGZO memristive readout neural network (non-volatile mode).A further step for the demonstration of the system should involve a crossbar design and the corresponding test.Further, it is worth noticing that IGZO memristors can be applied on flexible biocompatible substrates, such as polyimide with parylene as biofriendly encapsulation to be implemented in IoMT device application.
The IGZO precursor ink was optimized for inkjet printing while considering the Reynolds (Re), Webber (We) and Ohnesorge (Oh) numbers.The Ohnesorge number is a dimensionless value that describes the tendency for a drop to either stay together or fly apart, by comparing viscous forces with inertial and surface tension forces.The Ohnesorge number is related to the Reynolds number, and Weber number.The value of Z is defined as the inverse of Oh and used to evaluate the drop formation.For a stable drop formation, the value of Z must be between 1 and 10 68,69 .The IGZO ink has a viscosity of 4.16 cP at 20 °C (same temperature condition during printing) and a Z number of 4.8 (Table S1).The Figure S9 shows the Re, We and Oh values for the ink are inside the optimal area for a stable drop formation.The ink viscosity was measured using a Brookfield DV2T viscometer with a speed ranging from 1 to 50 rpm.

Device fabrication
The developed ITO/IGZO/Ag devices have a common bottom electrode structure.Figure 1a explains the fabrication of the IGZO memristors.The bottom electrode consists of ITO covered commercial glasses.The printing of the IGZO layer was carried out in a Dimatix DMP 2850 inkjet system using a piezoelectric multi-nozzle printing head from Dimatix (DMCLCP-16110) with 10 pL cartridge.The cartridge and stage temperature were kept at 25 °C and 50 °C, respectively.The frequency was 5 kHz and the drop spacing was set to 30 µm.The IGZO layer were printed with an area of 1000 × 1000 µm 2 followed by a post treatment at 200 °C for 1 h.Before the deposition of the top contacts, a 15 min UV/ozone surface activation was carried.As top contact, silver electrode was used due to facility of deposition with printing techniques 70,71 .Silver inks are very reliable and have a sintering temperature of 200 °C or lower, unlike other metal-based inks which need higher sintering temperatures to be conductive.Therefore, the silver nanoparticle colloidal ink (Sicryst I50T-13 from PV Nano Cell company)) was printed as two subsequent layers with an area of 250 × 250 µm 2 , on top of the IGZO layer by inkjet printing.
The device thickness was measured using a stylus XP-Plus 200 Stylus profilometer from Ambios.The surface morphology of the samples was also determined by Atomic Force Microscopy (AFM), with an Asylum MFP3D.The quasi-static current-voltage (I-V) characteristics and the pulse studies were measured using a Keithley www.nature.com/scientificreports/4200 SCS semiconductor analyser connected to the Janis ST-500 probe station.The signal was applied to the top electrode (Ag) while maintaining the bottom electrode (ITO) grounded.The speed of the measurements was at normal mode with a measurement rate of 50 mV/s without any delay time and the integration time was in auto setting.

Figure 1 .
Figure 1.(a) Schematic depicting the fabrication of printed Ag/IGZO/ITO memristors on glass substrate by inkjet printing technique (b) Average IGZO thickness in function of the number of printed layers with an optical microscope image of the printed Ag/IGZO/ITO memristors as inset.(c) I-V characteristics of Ag/IGZO/ ITO in function of the IGZO Thickness: 350 nm, 500 nm and 530 nm.(d) Read current at 100 mV on HRS and LRS in function of the IGZO thickness.(e) respective memory window.

Figure 3 .
Figure 3. Study of the mechanism of Ag/IGZO/ITO devices.Fitting of the SET curve: (a) on LRS for hopping and (b) on HRS for Schottky emission.Temperature measurements carried out in vacuum from 150 to 300 K with a step of 10 K on Ag/IGZO/ITO memristors: (c) SET sweep at 150 K and 300 K, (d) read current on LRS and HRS from 150 to 300 K, (e) Fitting mechanism of the LRS for hopping, (f) Fitting mechanism of the HRS for Schottky emission.

Figure 4 .
Figure 4. (a) Schematic of Ag/IGZO/ITO devices emphasizing the coexistence of two distinctive switching mechanisms.(b) Different I-V characteristics of the memristors taken from voltage sweeps: at larger current it follows a counter8wise switching (non-volatile) while for low current it follows an 8wise switching (volatile).(c) 100 cycles endurance voltage sweep for non-volatile programming.(d) 50 cycles endurance voltage sweep for volatile programming (e) Retention test for 105 s at 0.1 V (f) Current decay in IGZO memristor after being programmed by 1 write pulse (− 0.65 V, 0.1 ms); the current was then monitored by read pulses at 0.05 V for 25 ms.

Figure 5 .
Figure 5. (a) Current levels during the application of a − 0.65 V pulse every 60 ms.(b) Current decay for 25 ms after being applied different number of pulses (1, 5, 15) with its respecting fitting.(c) Time constant (τ) as a function of the number of pulses.(d) Read current taken at 0.05 V for 0.1 ms after applying a single pulse with an amplitude of − 0.65 V with different pulse intervals: 0.1 ms and 1 ms. https://doi.org/10.1038/s41598-024-58228-y